Lithium primary batteries using iron disulfide as a positive electrode active material (hereinafter simply referred to as a “lithium primary battery”) include a positive electrode material and a negative electrode material which have a very large theoretical capacity, i.e., iron disulfide in the positive electrode active material is about 894 mAh/g, and lithium in the negative electrode active material is about 3863 mAh/g. Thus, lithium primary batteries are very practical for a lightweight, high-capacity battery. Further, the initial open circuit voltage (OCV) of the lithium primary batteries is from 1.7 V to 1.8 V, and the average discharge voltage of the lithium primary batteries is about 1.5 V. Thus, the lithium primary batteries are very practical, also in terms of their compatibility with other 1.5 V-level primary batteries, such as a manganese battery or an alkaline manganese battery.
Cylindrical-shaped lithium primary batteries in practical use are configured such that an electrode group in which a positive electrode and a negative electrode are wound, with a separator interposed therebetween, is housed in a hollow cylindrical battery case. Thus, the area where the positive and negative electrodes face each other is larger than the other 1.5 V-level primary batteries. Therefore, the cylindrical-shaped lithium primary batteries have superior heavy-load discharge characteristics.
However, since the volume of the iron disulfide as a positive electrode active material increases as a result of a discharge reaction, the battery needs to include space for allowing for the volume expansion. For example, if there is no sufficient space in the battery, the separator may be compressed due to the volume expansion of the iron disulfide, and the electrolyte solution contained in the separator is squeezed out, resulting in depletion of the electrolyte solution between the positive and negative electrodes, or movement of ions may be restricted due to significant reduction of the space in the positive electrode mixture, and as a result, discharge polarization increases and it is not possible to exhibit superior discharge characteristics.
In view of the problems, Patent Document 1 discloses a technique in which an electrolyte layer is provided between the separator and an electrode to allow for the volume expansion of the iron disulfide, thereby preventing an increase in internal resistance.
However, it is practically difficult to provide such an electrolyte layer between the separator and an electrode in a cylindrical-shaped lithium primary battery in which a wound electrode group is housed.
Further, Patent Document 2 discloses a technique in which the volume expansion of the iron disulfide is reduced by making the capacity of the positive electrode larger than the capacity of the negative electrode.